Basic methods of measuring resistance. How to measure resistance

In this article we will try to learn how to measure small resistances. Radio amateurs sometimes have a need to accurately determine the resistance of a shunt when making or repairing an ammeter, so that it, in turn, also accurately displays its units of measurement or for other purposes. But how to do this when the multimeter does not have a milli-ohm scale, the markings are either absent or completely unknown and incomprehensible? Most measuring instruments have a minimum scale of 200 Ohms for measuring resistance and 3.5 - 4 digits, when you short-circuit the probes there is already about 0.7 Ohms, when measuring resistance 0.1 Ohms nothing changes, trouble. Let's fix it now.

I suggest using a bridge measurement circuit for this purpose. Everyone should understand what a bridge is; we won’t dwell on that. Let's make a bridge of resistors, apply some voltage to it and measure it, although we can also measure the current, it won't make a difference; we choose what is more accurate at hand. So what does low resistance measurement have to do with it? Patience, everything is in order from afar. There is such a wonderful thing as bridge balance. The product of the resistances of the opposite arms of the bridge, provided it is balanced, will be the same. And voltages and currents, when the bridge is balanced, will cancel each other out and give a total of 0.

(Let R0 be R3 and Rx be R4)

So, based on the above, if instead of one of the resistors we put our small resistance of an arbitrary value in the bridge, and make the other resistor variable or tuning (according to the diagram, we use two variable resistors to accurately balance the bridge, especially in the case when there are no multi-turn variables at hand resistors) to achieve bridge balance. This circuit can be used to measure shunts and small resistances:

It was lazy to assemble the circuit, especially since it takes a lot of time to make the board, so an experimental sample of the circuit was made by hanging installation. Here, resistors R1 and R2 are not 1%, but they were selected as close as possible to the resistance of a given value, the resistance error did not exceed 0.5% at room conditions.

But you need to know how to get the exact value of the measured resistance. Firstly, the main feature of such a circuit is that it “multiplies” the measured resistance. This means that there is no need for a milli-ohm scale in a multimeter. A resistance of 0.1 Ohm can already be measured on a kilo Ohm scale. Only the measurement will now be not direct, but indirect; you will have to use a little mathematics and calculate the final result of the measurement.

Let's decide what range of ratings we will measure (meaning low resistance or shunt resistance). To do this, you need to select the values ​​of the variable resistors:

According to the circuit, we use two variable resistors for greater interaction accuracy, 1 kOhm and 100 Ohm. This resistance of variable resistors will allow you to measure the maximum resistance of 1.1 ohms, the minimum while maintaining measurement accuracy of 0.01 ohms (with Rx = 0.01 ohms R0 should be 10 ohms, which also need to be measured quite accurately with your multimeter)

And the values ​​of constant resistors, so that the bridge can be easily balanced and it is convenient to calculate the value of the shunt or small resistance:

The multiplicity of resistors relative to each other is best taken exactly like this - 10, 100, 1000, in order to quickly calculate the final result, although no one forbids taking non-round numbers, so that you can also count with a calculator. According to the scheme, this is the ratio of 100,000 to 100, that is, a multiplier of 1000.

Let's put together a diagram. You can use any tuning or variable resistors, but for greater accuracy I advise you to take multi-turn tuning or variable resistors, and use constant ones with a tolerance of no more than 1%, or better yet, even less. The circuit uses a 9-volt Krona as a battery; it can be replaced with any other source. Capacitors in case of using power supplies for filtering. The circuit in our resistance configuration consumes 90 mA from a 9 V battery, so for frequent measurements, of course, it is more advisable to use a power supply. The circuit has been assembled, now we are studying the measurement technique. After connecting the measured resistance, it is necessary to apply voltage to the circuit, no matter what, but the higher it is, the greater the accuracy, set the meter to the limit of 200 mV and begin the process of balancing the bridge by rotating the trimming resistor until full zero appears on the voltmeter. This means that the bridge is balanced and all expressions are now valid for our circuit. Next, we measure the resistance of the tuning resistor and calculate the value of the small resistance:

or more beautifully like this

(219 Ohm * 100 Ohm)/100 kOhm we get 0.219 Ohm shunt resistance (see video).

Or, more simply, the result obtained must be divided by 1000 (since 100 kOhm/100 Ohm will be 1000 - our multiplier) in our case. So what do we see? Yes! This is the resistance that we measured 0.219 Ohm (~0.22 Ohm). Within the limits of good accuracy, and if you take into account errors in measurement and interaction with the circuit, it’s ideal.

Now you won’t have to rack your brains when the need for such measurements arises. The scheme is simple, but not many people know about it.

Attached to the article is a printed circuit board for making a mini multimeter attachment and a project for those who are curious to check this miracle, but are too lazy to assemble the circuit.

List of radioelements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
R1	Resistor	100 kOhm	1	1%		To notepad
R2	Resistor	100 Ohm	1	1%		To notepad
R0(1)	Trimmer resistor	1 kOhm	1	3296W		To notepad
R0(2)	Trimmer resistor	100 Ohm	1	3296W		To notepad
C1	Electrolytic capacitor	220 µF	1	Other denomination possible		To notepad
C2	Capacitor	100 nF	1

Resistance is a physical quantity that characterizes the properties of a body (object) to prevent the passage of electric current. To some extent, resistance is similar to the frictional force that occurs when a body moves along a certain surface. Resistance is measured in ohms (Ohms): 1 Ohm = 1 V (volts, voltage) / 1 A (amps, current). Resistance is measured using an ohmmeter or a digital or analog multimeter.

Steps

Measuring Resistance with a Digital Multimeter

1. Touch one terminal of the element with one probe, and touch the opposite terminal of the element with the second probe. Wait until the numbers on the indicator stop changing and write down the displayed number, which is the resistance value of the resistor.

   • For example, if the meter displays "0.6" and its upper right corner displays "MΩ", then the resistor value is 0.6 MΩ.

2. Turn off the multimeter. When you have finished measuring the resistance of the resistors, turn off the multimeter and disconnect the probes.

   Measuring resistance using an analog multimeter

   1. Select the element whose resistance you want to measure. To get an accurate result, measure the resistance of each element of the circuit (circuit). To do this, either remove the element from the circuit or measure the resistance before connecting the element to the circuit. Measuring the resistance of an element connected to a circuit may lead to inaccurate results due to the influence of other elements.

      Connect the multimeter probes to the appropriate connectors. Most multimeters have two probes - black and red, as well as several connectors that are designed to measure various quantities - resistance, voltage or current. As a rule, connectors designed to measure resistance are designated by the letters “COM” (English “common” - standard) and the Greek letter Ω (omega), which is a symbol of the unit of measurement ohm.

      • Connect the black lead to the jack labeled "COM" and the red lead to the jack labeled "Ohm".

   2. Turn on the multimeter and set the measurement range. The resistance of the element can range from several ohms (1 ohm) to several megaohms (1,000,000 ohms). For accurate results, set the resistance value range that matches the selected element. Some digital multimeters set this range automatically, while others do it manually. If you know what range the resistance of the selected element lies in, set the appropriate range; otherwise, determine the range by trial and error.

      • If you don't know the range, set the middle range first; Typically, this range is 0–20 kOhm.
      • Touch one terminal of the element (resistor) with one probe, and touch the opposite terminal of the element with the second probe.
      • The indicator arrow will begin to move along the scale and stop at a certain number, indicating the resistance value of the element.
      • If the needle moves toward the maximum range limit (left side), narrow the set range, reset the multimeter to zero (set the needle to zero), and repeat the measurement.
      • If the needle moves toward the minimum range limit (right side), expand the specified range, reset the multimeter to zero, and repeat the measurement.
      • Analog multimeters should be reset after each range change. To do this, touch one probe to another to cause a short circuit. If the needle does not reach zero, adjust its position using a special regulator (“Ohm regulator” or “Zero control”).

   3. Touch the leads of the multimeter to the terminals of the resistor whose resistance you want to measure. Touch one terminal of the element with one probe, and touch the opposite terminal of the element with the second probe. The arrow will begin to move from right to left - the minimum resistance value (right) is zero, and the maximum value (left) is 2000 Ohms (2 kOhms). An analog multimeter has multiple scales, so look for the resistance value on the scale labeled "Ω" (Ohm).

      • As the values ​​increase, the numbers on the scale will cluster closer together. Therefore, setting the correct range is critical to obtaining accurate readings.

   4. Definition of resistance. By touching the resistor terminals with the probes, the needle will stop somewhere in the middle of the scale. Make sure you read the value from the scale marked "Ω" (ohms); Write down the number that the arrow points to - it is the resistance value of the resistor.

      • For example, if the range you set is 0-10 ohms, and the arrow stops at the number 9, then the resistance of the element is 9 ohms.

   5. Set the maximum voltage range. When you finish using the multimeter, turn it off properly. To do this, set the voltage range to the maximum so you don't damage the device the next time you (or someone else) forgets to set the range in the first place. Turn off the multimeter and disconnect the probes.

   Getting accurate measurement results

   Measure resistance when elements are not connected to the circuit. If a resistor is connected to a circuit, then its resistance value will be inaccurate, since the multimeter measures not only the resistance of the resistor you need, but also the resistance of other resistors included in the circuit. However, sometimes it is necessary to measure the resistance of a resistor connected to a circuit.

   1. Measure the resistance of the de-energized element. The current passing through the circuit will negatively affect the accuracy of the multimeter readings, as it affects the resistance value of the resistors. In addition, additional voltage may damage the multimeter (so it is not recommended to measure the resistance of a battery or accumulator).

      • When measuring the resistance of a capacitor in a circuit, you must first discharge it. A discharged capacitor will be charged by the multimeter, which will lead to short-term jumps in the readings of the device.

During the manufacture, installation and operation of electrical and radio engineering devices and installations, it is necessary to measure electrical resistance.

In practice, various methods are used to measure resistance, depending on the nature of the objects and measurement conditions (for example, solid and liquid conductors, grounding conductors, electrical insulation); on requirements for accuracy and speed of measurement; on the value of the measured resistances.

Methods for measuring small resistances differ significantly from methods for measuring large resistances, since in the first case it is necessary to take measures to eliminate the influence of the resistance of connecting wires and transition contacts on the measurement results.

Measuring mechanisms of ohmmeters. For direct resistance measurement, single- and double-frame magnetoelectric measuring mechanisms are used.

A single frame mechanism can be used to measure resistances. For this purpose, an additional resistor with a constant resistance is introduced into the device

and supply it with a power source (for example, a dry cell battery). The resistance being measured is connected with the meter in series (Fig. 1) or in parallel.

When connected in series, the current in the meter , Where

- meter resistance; - power supply voltage.

Considering that

, Where - current sensitivity of the device (constant value), we find that the angle of deflection of the device needle at depends only on the value of the measured resistance:

If the scale is calibrated using this expression in units of resistance, then the device will be an ohmmeter. The voltage of dry elements decreases over time, so an error is introduced into the measurements, the greater the greater the actual voltage differs from the voltage at which the scale was calibrated.

An error from the variability of the supply source voltage does not occur if the measuring mechanism has two windings located on a common axis at a certain angle to each other (Fig. 2.).

Rice. 1. Fig. 2.

In a two-frame measuring mechanism, which is called a ratiometer, there are no counteracting springs; the rotating and counteracting moments are created by electromagnetic forces. Therefore, in the absence of current in the windings, the well-balanced moving part of the device is in indifferent equilibrium (the needle stops at any scale mark). When there is current in the coils, two electromagnetic moments directed in opposite directions act on the moving part.

The magnetic circuit of the measuring mechanism is designed so that the magnetic induction along the air gap is distributed unevenly, but in such a way that when the moving part is turned in any direction, the torque decreases and the counteracting moment increases (depending on the direction of rotation, the role of the moments changes).

The moving part stops when

or . It follows that the position of the arrow on the scale depends on the ratio of the currents in the windings, i.e. , but does not depend on the voltage of the supply source.

In the diagram fig. 2. It can be seen that the measured resistance

is included in the circuit of one of the logometer coils, therefore the current in it, as well as the deflection of the instrument needle, clearly depends on the value .

Using this dependence, the scale is calibrated in units of resistance and then the device is an ohmmeter. Ohmmeters for measuring insulation resistance are supplied with a power source with a voltage of up to 1000 V in order to carry out the measurement at a voltage approximately equal to the operating voltage of the installation. Such a source can be a built-in magnetoelectric generator with a manual drive or a transformer with a rectifier connected to the alternating current network.

Ohmmeters designed to measure high resistances (more than 1 MOhm) are called megaohmmeters.

Indirect methods for measuring resistance. The resistance of a resistor or other element of an electrical circuit can be determined from the readings of a voltmeter and ammeter (at constant current) using Ohm's law:

(diagrams Fig. 3, a, b). According to the diagram in Fig. 4 determine the resistance using the readings of one voltmeter. In switch position 1 P the voltmeter measures the network voltage, and in position 2 - voltage at the voltmeter terminals. In the latter case . From here

Indirect methods are used to measure medium resistances, and high resistances are also measured with one voltmeter. The accuracy of these methods depends significantly on the ratio of the values ​​of the measured resistance

and internal resistances of the ammeter and voltmeter. The measurement results can be considered satisfactory in accuracy if the following conditions are met: (see diagram Fig. 3, a); (see diagram Fig. 3, b); (see diagram Fig. 4).

Rice. 3 Fig. 4

Methods and comparison devices. To measure small and medium resistances, the method of comparing the measured resistance is used

with exemplary . These two resistances in the diagram in Fig. 5 are connected in series, so the current in them is the same. Its value is adjusted using a resistor so that it does not exceed the permissible current for resistances and . From here . Unknown voltage drops are measured with a voltmeter or potentiometer. The measurement results are more accurate if the resistances are of the same order, and the resistance of the voltmeter is large enough so that connecting it does not affect the mode of the main circuit.

When measuring small resistances using this method, the voltmeter is connected using potential clamps, which make it possible to exclude the resistance of the main circuit contacts from the measurement results.

In amateur radio or electrical engineering practice, sometimes there is a need to measure small resistances (less than 1 ohm), for example, when checking transformer windings, relay contacts, shunts, and grounding calculations. How to measure resistance of miliohms or even microohms? As you know, resistance measurement is based on converting their value into current or voltage. The work of the voltmeter attachment for measuring low resistances is based on this principle.

Milliohmmeter attachment

E That simple circuit from one foreign site is intended for measuring low resistance values ​​- from 0.001 to 1.999 ohms. "Direct Indication of Resistance, Ohms." You must use a separate battery to power it. The supply voltage is stabilized by the LM317LZ microcircuit. We recommend the small oneLM317LZ, but not LM317. But you can also use LM317 if you want. The trimmer must be adjusted exactly to 100 mA to obtain a highly accurate resistance measurement.

When measuring, try to reduce the length of the wires as much as possible, since every centimeter will provide additional resistance.

The display of a digital voltmeter (a regular D830 multimeter) will display a value in Ohms, from 0.001 to 1.999 Ohms. To test the device, measure several parallel-connected one-ohm resistors.

Analogue low resistance meter

You can assemble not just a console, but a ready-made independent device. This analog milliohmmeter uses two resistance measurement modes. At a stable current of 1A (scale 1 division = 0.002 Ohm) and at a stable current of 0.1A (scale 1 division = 0.02 Ohm). This is for the head shown in photo 1. As can be seen from the photo, the measuring head has a total deviation current of 100 μA. The price of a small division is 2 µA. At a current of 0.1A, the device will measure resistance from 0.02 Ohm to 1 Ohm. Those. the deviation of the arrow by the last division of the scale will correspond to one Ohm.

The principle of operation of the device is to measure the voltage drop across the measured resistance when a certain stable current passes through it. The frame resistance of the dial gauge is 1200 Ohms, the total deviation current is 0.0001 A, which means that if we use this indicator as a voltmeter, we will need to apply a voltage ofU = IxR = 0.0001x1200 = 0.12 V = 120 mV to deflect the arrow to the last division of the scale. This is exactly the voltage that should drop across a resistance of 1 Ohm at the measuring limit of the device from 0.02 Ohm to 1 Ohm. This means that at this measurement limit we need to pass a stable current of magnitude through the measured resistorI = U/R = 0.12/1 = 0.12A = 120 mA . The same can be calculated for another limit.

- an electrical quantity that characterizes the property of a material to prevent the flow of electric current. Depending on the type of material, the resistance can tend to zero - be minimal (miles/micro ohms - conductors, metals), or be very large (giga ohms - insulation, dielectrics). The reciprocal of electrical resistance is .

Unit electrical resistance - Ohm. It is designated by the letter R. The dependence of resistance on current in a closed circuit is determined.

Ohmmeter- a device for direct measurement of circuit resistance. Depending on the range of the measured value, they are divided into gigaohmmeters (for large resistances - when measuring insulation), and micro/miliohmmeters (for small resistances - when measuring transition resistances of contacts, motor windings, etc.).

There is a wide variety of ohmmeters by design from different manufacturers, from electromechanical to microelectronic. It is worth noting that a classic ohmmeter measures the active part of the resistance (so-called ohmics).

Any resistance (metal or semiconductor) in an alternating current circuit has an active and reactive component. The sum of active and reactive resistance is AC circuit impedance and is calculated by the formula:

where, Z is the total resistance of the alternating current circuit;

R is the active resistance of the alternating current circuit;

Xc is the capacitive reactance of the alternating current circuit;

(C - capacitance, w - angular speed of alternating current)

Xl is the inductive reactance of the alternating current circuit;

(L is inductance, w is the angular velocity of alternating current).

Active resistance- this is part of the total resistance of an electrical circuit, the energy of which is completely converted into other types of energy (mechanical, chemical, thermal). A distinctive property of the active component is the complete consumption of all electricity (no energy is returned to the network), and reactance returns part of the energy back to the network (a negative property of the reactive component).

The physical meaning of active resistance

Each environment where electric charges pass creates obstacles in their path (it is believed that these are nodes of the crystal lattice), into which they seem to hit and lose their energy, which is released in the form of heat.

Thus, a drop (loss of electrical energy) occurs, part of which is lost due to the internal resistance of the conducting medium.

The numerical value characterizing the ability of a material to prevent the passage of charges is called resistance. It is measured in Ohms (Ohm) and is inversely proportional to electrical conductivity.

Different elements of Mendeleev's periodic table have different electrical resistivities (p), for example, the smallest. Silver (0.016 Ohm*mm2/m), copper (0.0175 Ohm*mm2/m), gold (0.023) and aluminum (0.029) have resistance. They are used in industry as the main materials on which all electrical engineering and energy are built. Dielectrics, on the contrary, have a high shock value. resistance and are used for insulation.

The resistance of the conductive medium can vary significantly depending on the cross-section, temperature, magnitude and frequency of the current. In addition, different environments have different charge carriers (free electrons in metals, ions in electrolytes, “holes” in semiconductors), which are the determining factors of resistance.

Physical meaning of reactance

In coils and capacitors, when applied, energy accumulates in the form of magnetic and electric fields, which takes some time.

Magnetic fields in alternating current networks change following the changing direction of movement of charges, while providing additional resistance.

In addition, a stable phase and current shift occurs, and this leads to additional electricity losses.

Resistivity

How can we find out the resistance of a material if there is no flow through it and we do not have an ohmmeter? There is a special value for this - electrical resistivity of the material V

(these are tabular values ​​that are determined empirically for most metals). Using this value and the physical quantities of the material, we can calculate the resistance using the formula:

Where, p— resistivity (units ohm*m/mm2);

l—conductor length (m);

S - cross section (mm 2).